Comprehensive Understanding of Hillocks and Ion Tracks in Ceramics Irradiated with Swift Heavy Ions

Article Comprehensive Understanding of Hillocks and Ion Tracks in Ceramics Irradiated with Swift Heavy Ions

Norito Ishikawa 1,* , Tomitsugu Taguchi 2 and Hiroaki Ogawa 1

1 Nuclear Science and Engineering Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan; [email protected] 2 Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology (QST), Tokai, Ibaraki 319-1106, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-29-282-6089; Fax: +81-29-282-6716

Received: 13 November 2020; Accepted: 8 December 2020; Published: 9 December 2020

Abstract: Amorphizable ceramics (LiNbO3, ZrSiO4, and Gd3Ga5O12) were irradiated with 200 MeV Au ions at an oblique incidence angle, and the as-irradiated samples were observed by transmission electron microscopy (TEM). Ion tracks in amorphizable ceramics are confirmed to be homogenous along the ion paths. Magnified TEM images show the formation of bell-shaped hillocks. The ion track diameter and hillock diameter are similar for all the amorphizable ceramics, while there is a tendency for the hillocks to be slightly bigger than the ion tracks. For SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored, 200 MeV Au ion irradiation and TEM observation were also performed. The ion track diameters in these materials are found to be markedly smaller than the hillock diameters. The ion tracks in these materials exhibit inhomogeneity, which is similar to that reported for non-amorphizable ceramics. On the other hand, the hillocks appear to be amorphous, and the amorphous feature is in contrast to the crystalline feature of hillocks observed in non-amorphizable ceramics. No marked difference is recognized between the nanostructures in STO and those in Nb-STO. The material dependence of the nanostructure formation is explained in terms of the intricate recrystallization process.

Keywords: swift heavy ion; hillocks; ion tracks; ion irradiation; TEM

1. Introduction Long nanometer-sized damage trails are created in ceramics continuously along the trajectories of swift heavy ions (SHIs), if the energy transfer from a SHI to an electron system of ceramics is suﬃciently high [1–3]. Such characteristic damage is called an ion track. The mechanism of ion track formation has been extensively studied so far, and it has been one of the central topics in the research on ion-solid interactions. Ceramics can be categorized into two groups; amorphizable and non-amorphizable ceramics. If amorphous ion tracks are created, the ceramics are called amorphizable ceramics. The electronic stopping power dependence of ion track sizes in many amorphizable ceramics has been successfully predicted using the thermal spike model [4–8]. According to the model, the amorphous ion track formation is attributable to a local temperature rise suﬃcient to cause local melting along an ion path. On the other hand, there are non-amorphizable ceramics in which a crystal structure within an ion track region is not amorphized [9–14]. It has been found that in non-amorphizable ceramics, ion track size is markedly smaller than the size of the melt predicted by the thermal spike models [13,14]. Recent molecular dynamics (MD) simulation has revealed that the small ion tracks in non-amorphizable ceramics are attributable to fast recrystallization after transient melting [15–18]. It has been pointed out that the ionic nature of atomic bonding in non-amorphizable ceramics may be responsible for such fast recrystallization in non-amorphizable ceramics [9,19]. However, there is

Quantum Beam Sci. 2020, 4, 43; doi:10.3390/qubs4040043 www.mdpi.com/journal/qubs Quantum Beam Sci. 2020, 4, 43 2 of 14 currently no consensus on which material property makes the distinction between amorphizable and non-amorphizable ceramics. Therefore, it is important to examine the material dependence of ion track formation in terms of amorphization/recrystallization. Ion track formation caused by SHIs is often accompanied by the formation of hillocks (so-called surface ion tracks) [20–35]. Our recent studies revealed that both ion tracks and hillocks are amorphous in the case of amorphizable ceramics (Y3Fe5O12 (YIG)) [36,37], whereas crystalline hillocks are found in the case of non-amorphizable ceramics (CaF2, SrF2, BaF2, and CeO2)[36,38]. Since the surface protrusion is a direct consequence of local melting along the ion path (melting of the ion track region), the observation of crystalline hillocks in non-amorphizable ceramics also provides a strong evidence of recrystallization after melting of the ion track region. It was also found that the hillock diameter always coincides with the diameter of a melt predicted by the thermal spike model for both amorphizable and non-amorphizable ceramics. This means that a hillock diameter value is affected by a melting process, but it remains unchanged after the subsequent recrystallization process. It is likely that a hillock size reflects melting, whereas an ion track size reflects both melting and subsequent recrystallization. Therefore, a comparative analysis of ion tracks and hillocks allows us to elucidate the whole processes of melting and amorphization/recrystallization. Moreover, there must be a variety of hillock and ion track morphologies due to an intricate recrystallization process, which can be the origin of the material dependence of nanostructure formation. We recently proposed a method for precise measurement of a hillock size by transmission electron microscopy (TEM) [36–38]. The method is useful for the direct observation of a hillock side-view. It allows the accurate measurement of hillock dimensions and the identification of hillock crystal structure. In our previous study, we have studied hillocks and ion tracks in non-amorphizable ceramics such as CaF2, SrF2, and BaF2, whereas only one amorphizable ceramic (YIG) has been studied [36,37]. In the present study, we have further studied the relationship between the hillock diameter and ion track diameter for various types of ceramics. First, we show results of the TEM observations for SHI-irradiated amorphizable ceramics such as LiNbO3, ZrSiO4, and Gd3Ga5O12 (GGG) and then for SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO), whose hillock formation has not been fully explored. The electrical resistivity of STO increases by more than nine orders of magnitude by doping with 0.5 wt% Nb, whereas its crystal structure remains unaffected by the doping [39]. The effect of Nb doping on its hillock morphology is also investigated in this study. Based on the TEM investigations of both amorphizable and non-amorphizable ceramics, we discuss how the SHI-induced nanostructure formation depends on the intricate recrystallization process.

2. Experiments Thin samples for TEM observations were prepared before irradiation by the following procedure. The original materials were ZrSiO4 (98%) powder (Kojundo Chemical Laboratory Co., Ltd., Saitama, Japan), LiNbO3 single crystals (Onizawa Fine Product Co., Ltd., Ibaraki, Japan), Gd3Ga5O12 (GGG) single crystals (Rare Metallic Co., Ltd., Tokyo, Japan), SrTiO3 (STO) single crystals (Shinkosha Co., Ltd., Kanagawa, Japan), and 0.5 wt% niobium-doped STO (Nb-STO) single crystals (Shinkosha Co., Ltd., Kanagawa, Japan). The original materials were finely ground using an agate mortar and pestle. For the preparation of LiNbO3 specimens, first, 3 mm diameter nickel grids with 2000 lines/inch were coated using 0.5% Neoprene W in toluene (Nisshin EM Co., Ltd., Tokyo, Japan), which works as an adhesive agent, and then the ground LiNbO3 powder was randomly dispersed on the grids. For the preparation of materials other than LiNbO3, the ground samples were dispersed in ethanol by an ultrasonic bath for a few minutes, and then the ethanol was dropped on 3 mm diameter 200 mesh copper grids covered with porous carbon films. The grids were then air-dried at room temperature. The samples on the 32+ grids were subsequently irradiated with 200 MeV Au ions at an oblique incidence angle (45◦ relative to normal direction) at room temperature in a tandem accelerator at JAEA-Tokai (Japan Atomic Energy Agency, Tokai Research and Development Center, Tokai, Japan). The charge state (32+) was chosen to ensure the charge of the incident ions to have the average value of the equilibrium charge. The samples Quantum Beam Sci. 2020, 4, 43 3 of 14

Quantum Beam Sci. 2020, 4, x FOR PEER REVIEW 3 of 15 were irradiated with ions at 1 1011 ions/cm2. The as-irradiated samples were examined using a × JEOLtransmission Ltd., Tokyo, electron Japan) microscope operated (TEM,at 200 kV. Model The 2100F, ion track JEOL size Ltd., was Tokyo, measured Japan) using operated the TEM at images 200 kV. takenThe ion at tracka low size magnification, was measured wherein using clear the TEM line-like images contrasts taken atwere a low expected magniﬁcation, to be imaged. wherein On clear the otherline-like hand, contrasts the hillock were expected size was to measured be imaged. using On the the other TEM hand, images the hillocktaken at size a washigh measuredmagnification, using whereinthe TEM a images clear contour taken at of a highhillocks magniﬁcation, was expected wherein to be obtained. a clear contour The electronic of hillocks stopping was expected power to (S bee) wasobtained. estimated The electronicusing SRIM-2008 stopping [40,41]. power (Se) was estimated using SRIM-2008 [40,41].

3. Results Results and and Discussions Discussions

3.1. Dimensions Dimensions of of Hillocks and Ion Tracks in LiNbO3,, ZrSiO ZrSiO4,, and and GGG GGG We havehave observedobserved nanostructures nanostructures created created by by SHI SHI irradiation irradiation in amorphizable in amorphizable ceramics ceramics using using TEM. TEM.Figure Figure1a–c show 1a–c the show bright the field bright images field of images ion tracks of ion in LiNbO tracks3 ,in ZrSiO LiNbO4, and3, ZrSiO Gd3Ga4, and5O12 Gdirradiated3Ga5O12 withirradiated 200 MeV with Au 200 at MeV an oblique Au at an incidence oblique angle. incidence In the angle. figures, In the the figures, ion tracks theare ion imaged tracks are as line-likeimaged ascontrasts. line-like Thecontrasts. diameter The of diameter the ion of tracks the ion can trac beks estimated can be estimated by measuring by measuring the width the of width the line-like of the line-likecontrasts. contrasts. However, However, for most offor the most ion of track the ion images, track it images, is difficult it is todifficult distinguish to distinguish the ion track the ion contrasts track contrastsfrom hillock from contrasts. hillock contrasts. However, However, if the hillocks if the are hillo createdcks are at thecreated edge at of the the edge samples, of the it samples, is possible it tois possibleinfer the to hillock infer side-view.the hillock Itside-view. can be concluded It can be concluded from the images from the that images hillocks that have hillocks similar have diameters similar diametersto those of to ion those tracks. of ion Size tracks. distribution Size distribution of ion tracks of ion is tracks shown is inshown Figure in2 Figure. The average 2. The average sizes of sizes the ofhillocks the hillocks and ion and tracks ion tracks are summarized are summarized in Table in1 Table. 1.

(a) (b)

(c)

Figure 1. Bright field ﬁeld images of ionion trackstracks inducedinduced inin (a(a)) LiNbO LiNbO3,(3, b(b)) ZrSiOZrSiO44,, andand (c(c)) Gd Gd3Ga3Ga55OO1212 irradiated with with 200 200 MeV MeV Au Au32+32 at+ atan an oblique oblique incidence incidence angle. angle. The The images images were were taken taken at relatively at relatively low magnification.low magniﬁcation.

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1 1 Track diameter Track diameter LiNbO3 ZrSiO4 Hillock diameter Hillock diameter Hillock height Hillock height

0.5 0.5 Frequency Frequency

0 0 0102001020 Size (nm) Size (nm) (a) (b) 1 Track diameter GGG Hillock diameter Hillock height

0.5 Frequency

0 01020 Size (nm) (c)

Figure 2. Size distribution of the track diameter, the hillock diameter, and the hillock height in (a) Figure 2. Size distribution of the track diameter, the hillock diameter, and the hillock height in (a) LiNbO3, LiNbO3, (b) ZrSiO4, and (c) Gd3Ga5O12 irradiated with 200 MeV32+ Au32+ at an oblique incidence angle. (b) ZrSiO4, and (c) Gd3Ga5O12 irradiated with 200 MeV Au at an oblique incidence angle.

Table 1.1. Average diameterdiameter ofof ionion tracks tracks (D (Dtracktrack), averageaverage diameterdiameter ofof hillockshillocks (D (Dhillockhillock),), average average height height

of hillocks (Hhillock) )are are listed listed with with standard standard deviations deviations for for LiNbO LiNbO3,, ZrSiO ZrSiO44, ,and and GGG GGG irradiated irradiated with with 32+ 200 MeV Au 32+. .The The corresponding corresponding S Se evaluesvalues are are also also listed. listed.

DDtracktrack (nm)(nm) Dhillock Dhillock (nm)(nm) Hhillock Hhillock (nm)(nm) Se (keV/nm) Se (keV/nm) LiNbOLiNbO3 11.711.7 ± 1.31.3 13.2 13.2 ± 1.51.5 5.8 ± 5.8 1.2 1.2 28.1 28.1 3 ± ± ± ZrSiO 9.5 0.5 11.6 1.1 5.3 0.8 29.6 ZrSiO4 4 9.5 ±± 0.5 11.6 ± 1.1± 5.3 ± 0.8± 29.6 GGG 11.3 0.9 14.7 2.0 4.7 1.0 34.2 GGG 11.3 ±± 0.9 14.7 ± 2.0± 4.7 ± 1.0± 34.2

Previous literaturesliteratures have have reported reported ion ion track track sizes sizes in LiNbO in LiNbO3 [42–345 [42–45],], ZrSiO ZrSiO4 [46,474 ],[46,47], and GGG and [ 45GGG,48]. [45,48].They have They confirmed have confirmed that the ionthat tracks the ion in tracks these ceramicsin these ceramics are amorphous. are amorphous. A good summary A good summary of the ion oftrack the dataion track in LiNbO data 3inis LiNbO given in3 is Ref. given [45 in], inRef. which [45], thein which dependence the dependence of ion track ofsize ion ontrack the size electronic on the electronicstopping power stopping (Se) ispower shown. (S Thee) is Sshown.e-dependence The S demonstratese-dependence that demonstrates an ion track that size dependsan ion track primarily size dependson the electronic primarily stopping on the power,electronic whereas stopping low power, ion velocity whereas acts aslow a secondaryion velocity factor acts thatas a contributessecondary factorto a bigger that ioncontributes track owing to a tobigger the velocity ion track effect owin [49g]. to The the literature velocity also effect demonstrates [49]. The literature that ion trackalso demonstratessizes are in accordance that ion withtrack thesizes values are in predicted accordance by the with thermal the values spike model.predicted The by present the thermal study showsspike model.that the The ion trackpresent diameter study shows in LiNbO that irradiatedthe ion track with diameter 200 MeV in Au LiNbO (Se =3 28.1irradiated keV/nm) with is 11.7200 MeV1.3 nm,Au 3 ± whereas(Se = 28.1 thekeV/nm) previous is 11.7 study ± 1.3 showed nm, whereas that the the ion previous track diameter study showed in LiNbO that3 irradiated the ion track with diameter 201 MeV in U LiNbO3 irradiated with 201 MeV U ions (Se = 28.1 keV/nm) was around 13 nm [44,45], indicating that the present study is in accordance with the previous results within the experimental error.

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ions (Se = 28.1 keV/nm) was around 13 nm [44,45], indicating that the present study is in accordance with the previous results within the experimental error. Previous studies reported that the ion track diameter for ZrSiO4 irradiated with 10 GeV Pb ions (Se = 20.0 keV/nm) was 5.2 0.2 nm [46] and that for ZrSiO irradiated with 2.9 GeV Pb ions ± 4 (Se = 33.6 keV/nm) [47] was 8 nm. The present study shows that the ion track diameter for ZrSiO4 irradiated with 200 MeV Au ions is 9.5 0.5 nm, exhibiting large ion tracks. Since the large ion ± tracks observedQuantum Beam in theSci. 2020 present, 4, x FOR studyPEER REVIEW could be due to the relatively high electronic stopping 5 of 15 power (S = 29.6 keV/nm) and the velocity effect [49], there is no contradiction between the present and e Previous studies reported that the ion track diameter for ZrSiO4 irradiated with 10 GeV Pb ions previous results.(Se = 20.0 keV/nm) was 5.2 ± 0.2 nm [46] and that for ZrSiO4 irradiated with 2.9 GeV Pb ions (Se = 33.6 A goodkeV/nm) summary [47] was of8 nm. the The previous present study ion trackshows that data the in ion GGG track isdiameter given for in ZrSiO Ref.4 [irradiated45]. The present with 200 MeV Au ions is 9.5 ± 0.5 nm, exhibiting large ion tracks. Since the large ion tracks observed study shows that the ion track diameter for GGG irradiated with 200 MeV Au (Se = 34.0 keV/nm) in the present study could be due to the relatively high electronic stopping power (Se = 29.6 keV/nm) is 11.3 0.9 nm, whereas a previous study shows a similar ion track diameter (12.4 1.6 nm for GGG ± and the velocity effect [49], there is no contradiction between the present and previous results.± irradiated withA good 250 MeVsummary Pb of ions the previous (Se = 34.0 ion keVtrack/ nm))data in [ 45GGG]. Therefore,is given in Ref. the [45]. present The present result study is consistent with the previousshows that results.the ion track The diameter present for resultsGGG irradiated are also with consistent 200 MeV Au with (Se = the34.0 keV/nm) prediction is 11.3 made ± by the thermal spike0.9 nm, model. whereas a previous study shows a similar ion track diameter (12.4 ± 1.6 nm for GGG irradiated with 250 MeV Pb ions (Se = 34.0 keV/nm)) [45]. Therefore, the present result is consistent with the Figureprevious3a–c showresults. magnifiedThe present results images are also of consis hillockstent with in LiNbOthe prediction3, ZrSiO made 4by, the and thermal GGG, spike respectively, irradiatedmodel. with 200 MeV Au at an oblique incidence angle. As shown in the figures, hillocks are successfully observedFigure 3a–c in show those magnified materials. images It is alsoof hillocks found in that LiNbO the3, hillocksZrSiO4, and are GGG, clearly respectively, amorphous, which confirms theirradiated amorphizable with 200 MeV nature Au at of an these oblique ceramics. incidenceThe angle. side-view As shown ofin thethe figures, hillocks hillocks allows are measuring successfully observed in those materials. It is also found that the hillocks are clearly amorphous, both hillockwhich diameter confirms and the amorphizable height. The nature distribution of these ofceramics. the hillock The side-view sizes is of shown the hillocks in Figure allows2 together with that ofmeasuring the ion both track hillock sizes. diameter The averageand height. sizes The distribution of the hillocks of the hillock and track sizes is sizes shown are in summarizedFigure in Table1. As2 together demonstrated with that inof thethe ion table, track the sizes. hillock The average diameter sizes isof alwaysthe hillocks similar and track to the sizes ion are track size, although thesummarized former tendsin Table to 1. be Asslightly demonstrated larger in thanthe tabl thee, the latter. hillock A hillockdiameter height is always seems similar nearly to the half of the ion track size, although the former tends to be slightly larger than the latter. A hillock height seems hillock diameter.nearly half of the hillock diameter.

(a) (b)

(c)

Figure 3. Bright field images of hillocks induced in (a) LiNbO3, (b) ZrSiO4, and (c) Gd3Ga5O12 Figure 3. Bright ﬁeld images of hillocks induced in (a) LiNbO3,(b) ZrSiO4, and (c) Gd3Ga5O12 irradiated with 200 MeV Au32+ at an oblique incidence angle. The images were taken at relatively high 32+ irradiatedmagnification. with 200 MeV Au at an oblique incidence angle. The images were taken at relatively high magniﬁcation.

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Although the hillock shape seems to be semi-spherical in the low magnification images, the Although the hillock shape seems to be semi-spherical in the low magniﬁcation images, magnified images demonstrate the formation of bell-shaped hillocks. Bell-shaped hillocks can also be the magniﬁed images demonstrate the formation of bell-shaped hillocks. Bell-shaped hillocks can also found in Y3Fe5O12 [36,37] and Y3Al5O12 [16], that are also categorized as amorphizable ceramics. be found in Y3Fe5O12 [36,37] and Y3Al5O12 [16], that are also categorized as amorphizable ceramics.

3.2. Formation Process of Hillocks and Ion Tracks in LiNbO3, ZrSiO4, and GGG 3.2. Formation Process of Hillocks and Ion Tracks in LiNbO3, ZrSiO4, and GGG The present study shows that both hillocks and ion tracks exhibit amorphous features, and they have similar sizes in amorphizable ceramics (LiNbO3, ZrSiO4, and GGG). The present results agree have similar sizes in amorphizable ceramics (LiNbO3, ZrSiO4, and GGG). The present results agree withwith thethe predictionprediction ofof thethe thermalthermal spikespike model.model. According to the thermal spike model, SHIs causecause transient meltingmelting along the ion path, when the electronic stopping power exceeds a certain threshold value. During such transient melting,melting, thermal pressure and volume change causedcaused byby solid-liquidsolid-liquid transition lead toto surfacesurface protrusionprotrusion ofof thethe melt.melt. In thethe amorphizableamorphizable ceramics,ceramics, rapidrapid coolingcooling afterafter melting results inin thethe formationformation ofof amorphousamorphous ionion trackstracks andand hillocks.hillocks. It isis importantimportant to notenote that,that, althoughalthough the hillock diameter appears to be similar to the ion track diameter, the magniﬁedmagnified TEM images shows that the fo formerrmer is always slightly larger larger than than the the latter. latter. This can be ascribed toto the spreading tendencytendency ofof aa liquidliquid onon aa solidsolid surface. It is likely that the hillock shape is determined by the balance of the adhesiveadhesive (the(the liquidliquid wantingwanting to maintainmaintain contactcontact with the solid) and cohesive forces within the liquid (both internal cohesive force and surface tension) during melting. If the adhesive force dominates, the protru protrudedded part of the melt can spread over the surface, leadingleading toto thethe formationformation ofof bell-shapedbell-shaped hillocks. Conv Conversely,ersely, it can turn into a spherical shape, if the cohesive forceforce dominates. dominates. Such Such spherical spherical hillocks hillocks have have been been reported reported in some in some ceramics ceramics irradiated irradiated with with high-energy fullerene ions having a very high Se [50,51]. Large volume of surface protrusion high-energy fullerene ions having a very high Se [50,51]. Large volume of surface protrusion induced induced by high Se may be closely related to the formation of spherical hillocks. by high Se may be closely related to the formation of spherical hillocks.

3.3. Hillocks and Ion Tracks inin SrTiOSrTiO33 and Nb-Doped SrTiOSrTiO33 Figures4 4 and and5 5show show the the bright bright ﬁeld field image image of of the the ion ion tracks tracks in in STO STO and and Nb-STO Nb-STO irradiated irradiated with with 200 MeVMeV AuAu atat anan obliqueoblique incidence incidence angle, angle, respectively. respectively. The The ion ion track track has has a brighta bright core core surrounded surrounded by aby dark a dark fringe fringe in the in underfocus the underfocus condition condition (Figures (Figures4a and5 a),4a whereasand 5a), it haswhereas a dark it corehas surroundeda dark core bysurrounded a bright fringe by a bright in the fringe overfocus in the condition overfocus (Figures condition4b and(Figures5b). The 4b and Fresnel 5b). fringeThe Fresnel is an indicationfringe is an ofindication the formation of the formation of ion tracks of ion with tracks a lower with densitya lower thandensity that than of the that surrounding of the surrounding matrix [matrix52,53]. Such[52,53]. focus-dependent Such focus-dependent Fresnel contrastFresnel contrast is not found is not in found amorphizable in amorphizable ceramics ceramics (e.g., YIG, (e.g., LiNbO YIG,3, ZrSiOLiNbO4,3, and ZrSiO GGG).4, and GGG).

(a) (b)

3232++ Figure 4.4. Bright ﬁeldfield imagesimages ofof ionion trackstracks inducedinduced inin SrTiOSrTiO33 irradiatedirradiated withwith 200200 MeVMeVAu Au at an oblique incidence angle. The images were taken in (a)) underfocusunderfocus andand ((b)) overfocusoverfocus conditions.conditions.

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(a) (b)

3 32+ Figure 5. BrightBright field ﬁeld images images of ofion ion tracks tracks induced induced in Nb-doped in Nb-doped SrTiO SrTiO3 irradiated3 irradiated with with200 MeV 200 Au MeV Auat an32+ obliqueat an obliqueincidence incidence angle. The angle. images The were images taken werein (a) takenunderfocus in (a) condition underfocus and condition (b) overfocus and (condition.b) overfocus condition.

The ion tracks in STO and Nb-STONb-STO havehave inhomogeneousinhomogeneous morphology in contrast to the homogenous morphology morphology of of the the ion ion tracks tracks in in amor amorphizablephizable ceramics. ceramics. The The inhomogeneity inhomogeneity of the of ion the tracksion tracks is a common is a common feature feature observed observed in non-amorphizable in non-amorphizable ceramics ceramics such as suchCeO2 as[38], CeO CaF2 2[,38 SrF],2 CaF, and2, BaFSrF22, and[36], BaF in 2which[36], in partial which partialrecrystallization recrystallization after aftertransient transient melting melting is the is the likely likely cause cause of of the inhomogeneous ion track formation. Black Black contrast contrastss corresponding corresponding to to hillocks hillocks are found at the end of the ion tracks. Figures 66 andand7 7 show show magniﬁed magnified images images of of a a hillock hillock created created at at the the edge edge of of the the thin thin TEM samplessamples ofof STO STO and and Nb-STO, Nb-STO, respectively. respectively. Bell-shaped Bell-shaped hillocks hillocks are are observed observed in STO in STO and and Nb-STO. Nb- STO.In both In materials,both materials, hillocks hillocks are found are found to be amorphous. to be amorphous. It is interesting It is interestin to ﬁndg thatto find amorphous that amorphous hillocks hillocksare created, are created, although although the ion track the ion region track is region recrystallized. is recrystallized.

Figure 6. Bright field images of hillocks induced in SrTiO3 irradiated with 200 MeV Au3232++ at an oblique Figure 6. Bright field ﬁeld images of hillocks inducedinduced inin SrTiOSrTiO33 irradiatedirradiated withwith 200 200 MeV MeV Au Au at an oblique incidence angle.

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3232++ Figure 7. Bright ﬁeldfield images of hillockshillocks inducedinduced inin Nb-dopedNb-doped SrTiOSrTiO33 irradiatedirradiated withwith 200200 MeVMeV AuAu at an oblique incidence angle.

Since itit isis di difficultfficult to to find find inhomogeneous inhomogeneous ion tracksion tracks in TEM, in weTEM, could we notcould find not suffi findcient sufficient numbers numbersof ion tracks of ion and tracks hillocks and to hillocks perform to aperform statistical a statistical analysis. Therefore,analysis. Therefore, in this study, in this we study, only show we only the showsize range the size of ion range tracks of and ion hillocks tracks observedand hillocks in STO observed and Nb-STO in STO in and Table Nb-STO2. As demonstrated in Table 2. As in demonstratedthe table, there in is the a markedtable, there diff erenceis a marked in the differen diametersce in of the ion diameters tracks and of hillocksion tracks in and both hillocks STO and in bothNb-STO. STO Theand largeNb-STO. hillock The diameterlarge hillo comparedck diameter with compared the ion with track the diameter ion track is diameter also found is also in typical found innon-amorphizable typical non-amorphizable ceramics (CaF ceramics2, SrF2 ,(CaF and2, BaF SrF2).2, Theand presentBaF2). The result present can be result a strong can evidence be a strong that evidencesupports thethat recrystallization supports the recrystallization of the ion track of regions the ion in track STO regions and Nb-STO. in STO Conversely, and Nb-STO. the Conversely, amorphous thehillocks amorphous are signs hillocks of the are failure signs of of recrystallization. the failure of recrystallization. It seems that STO It seems and Nb-STOthat STO areand intermediate Nb-STO are intermediateceramics between ceramics amorphizable between amorphizable and non-amorphizable and non-amorphizable ceramics. ceramics. Here, it is important to discuss whether the ion tracks are actually crystalline or not, since some of theTable previous 2. Approximate studies claimed values that of ion they track are diameter amorphous (Dtrack in), hillockSTO. According diameter (D tohillock Ref.), [54], and hillockanalysis of X-rayheight diffraction (Hhillock peaks) in SrTiO in ion-irradiated3 (STO) and 0.5 STO wt% supports niobium-doped creation STO of amorphous (Nb-STO). The ion corresponding tracks due to S singlee impacts.values In arethe also same listed. literature, although crystalline tracks containing defects are observed by TEM, the authors claimed that the crystalline tracks are created owing to 200 keV electron beam exposure Dtrack (nm) Dhillock (nm) Hhillock (nm) Se (keV/nm) which can cause amorphous–crystalline transition of ion track areas. Conversely, our TEM results of STO 3~5 12~15 4~5 28.6 STO and Nb-STONb-STO support creation 3~4 of crystalline 11~13 ion tracks rather 4~5 than amorphous 28.5 ion tracks. For example, an inhomogenous feature of ion tracks in STO and Nb-STO is similar to that observed in partially recrystallized ion tracks in non-amorphizable ceramics. Moreover, the markedly smaller ion Here, it is important to discuss whether the ion tracks are actually crystalline or not, since some of track diameter than the hillock diameter demonstrates that the ion track region is partially the previous studies claimed that they are amorphous in STO. According to Ref. [54], analysis of X-ray recrystallized after transient melting. diffraction peaks in ion-irradiated STO supports creation of amorphous ion tracks due to single impacts. In the same literature, although crystalline tracks containing defects are observed by TEM, the authors Table 2. Approximate values of ion track diameter (Dtrack), hillock diameter (Dhillock), and hillock height claimed that the crystalline tracks are created owing to 200 keV electron beam exposure which can (Hhillock) in SrTiO3 (STO) and 0.5 wt% niobium-doped STO (Nb-STO). The corresponding Se values are causealso amorphous–crystalline listed. transition of ion track areas. Conversely, our TEM results of STO and Nb-STO support creation of crystalline ion tracks rather than amorphous ion tracks. For example, an inhomogenous featureD oftrack ion (nm) tracks in Dhillock STO (nm) and Nb-STO Hhillock is (nm) similar to Se that (keV/nm) observed in partially recrystallized ionSTO tracks in non-amorphizable3~5 12~15 ceramics. Moreover, 4~5 the markedly 28.6 smaller ion track diameter thanNb-STO the hillock diameter3~4 demonstrates 11~13 that the ion 4~5 track region is 28.5 partially recrystallized after transient melting. The clear difference in the morphology between the hillocks in STO and Nb-STO was not observed in this study. Therefore, the influence of Nb-doping on the hillock formation is not found in this study. A previous study using TEM and small angle X-ray scattering (SAXS) reported that 1

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The clear difference in the morphology between the hillocks in STO and Nb-STO was not observed in this study. Therefore, the influence of Nb-doping on the hillock formation is not found in this study. A previous study using TEM and small angle X-ray scattering (SAXS) reported that 1 wt% Nb-doping does not affect track formation [55]. Track formation is easy in insulators since the energy of hot electrons is transferred to the lattice in insulators before being cooled down by free electrons. In contrast, track formation is difficult in metals since high electronic conductivity allows rapid energy diffusion in an electron subsystem. Although the electrical conductivity of Nb-STO is more than nine orders of magnitude higher than that of STO, the electron density may still be too low to influence the formation of hillocks and ion tracks. It should be noted that the electrical conductivity of metals is still much higher than that of Nb-STO. The difference of bonding character (metallic bonding in metals vs. ionic/covalent bonding in STO) should be also contributing to the less sensitivity of track formation in metals [55].

3.4. Factors That Determine the Recrystallization Process The presence of recrystallization process separates non-amorphizable ceramics from amorphizable ceramics. Two factors that determine the recrystallization process have been proposed; (1) simplicity of lattice structure and (2) the strength of ionic bonding. The previous literature [15] proposed that a simple lattice structure with the smallest number of the peaks in the pair correlation function is in correlation with the recrystallization efficiency. Actually, amorphous ion tracks and hillocks are observed in YIG and GGG that have complicated crystal structure, whereas crystalline ion tracks and hillocks are observed in rather simple structured compounds (CaF2, SrF2, BaF2, and CeO2 with fluorite structure). STO has a relatively simple cubic structure with space group of Pm3m, where there are only 5 atoms in the unit cell. The relatively simple crystal structure of STO can be the origin of the recrystallization of the ion track region. However, creation of amorphous hillocks in STO demonstrates that STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics. Here, it is worth noting that STO has a perovskite structure which is composed of TiO2 planes and SrO planes, which make the structure somehow complicated. This unique structure of STO seems to be responsible for the intermediate behavior of ion-induced nanostructure formation. Therefore, we believe that the correlation of recrystallization effectiveness with the simplicity of the crystal structure is one of the promising hypotheses, and its validity should be further examined. Another possible factor is the strength of ionic bonding, i.e., materials with a higher degree of ionicity recrystallize easily [19,56]. Non-amorphizable ceramics include many ionic crystals. It is conceivable that long-range ionic forces rather than short-range covalent interaction contribute to rapid recrystallization [19,57]. STO has mixed ionic-covalent bonding properties. While a hybridization of the O-2p states with the Ti-3d states within the TiO6 octahedra leads to a pronounced covalent bonding, Sr2+ and O2- ions exhibit an ionic bonding character. Although STO is one of the complex oxides, the ionic bonding character in STO may be also responsible for the partial recrystallization of ion tracks. Therefore, the two factors mentioned above is important to explain why STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics. It is worth discussing why the hillock region fail to recrystallize, whereas the ion track region recrystallizes in STO. The previous MD simulation demonstrated that oxygen atoms settle in their original sites faster than metal atoms in SHI-irradiated MgO and Al2O3, and metallic atoms then adjust to a layer of oxygen that has already been built [15]. The study concluded that that the ability of oxygen atoms to reach their equilibrium sites governs the recrystallization process after melting. It is conceivable that in STO, oxygen loss in the hillock region effectively hinders recovery of the lattice structure. The hillock region tends to be more oxygen deficient than the ion track region, since oxygen atoms tend to escape from the irradiated surface. An important role of oxygen deficiencies should be further investigated in the future. Quantum Beam Sci. 2020, 4, 43 10 of 14

3.5. Summary of Formation Processes of Hillocks and Ion Tracks To understand the difference between nanostructure formation in amorphizable and non-amorphizable ceramics, it is important to describe the process in terms of melting and successive recrystallization. It is convenient to start the discussion with CeO2, which is one of the non-amorphizable ceramics. According to Ref. [38], hillocks are found to be spherical in CeO2 irradiated with 200 MeV Au. It was found that hillocks (spherical objects) have crystalline features, where the lattice orientation of hillocks was aligned with that of the matrix. This points out to the process consisting of the following three steps as explained in Figure8a. (1) A molten region is created along the ion path. A part of the molten region protrudes above the surface because of the thermal pressure and additional pressure due to volume change caused by solid-liquid transition. (2) During cooling, the molten region embedded in the matrix begins to recrystallize. The partial recrystallization results in ion tracks smaller than those expected from the size of the melt. The shape change (spheroidization) of the protruded part strongly suggests that the protruded part remains liquid for a long period of time which is long enough for the molten protrusion to change its shape. The protruded part of the molten region can spheroidize if the surface tension is strong enough. (3) A droplet at the surface starts to recrystallize epitaxially using the matrix as a template lattice, so that a crystalline nanosphere having the same crystal orientation as that of the matrix is formed. This result strongly suggests that the molten region embedded in the matrix is solidified before the protruded part of the melt is solidified. It is reasonable to assume that this sequence of the solidification process applies to all SHI-irradiated ceramics. Such a sequence of Quantumsolidification Beam Sci. process 2020, 4, x is FOR supported PEER REVIEW by the MD simulation of the nanostructure formation in CaF 112 of[16 15].

FigureFigure 8. Schematic formationformation processesprocesses of of ion ion tracks tracks and and hillocks hillocks in (ain) CeO(a) CeO2,(b2), fluorides(b) fluorides (CaF 2(CaF, SrF22, , SrFand2, BaFand2 ),BaF (c)2 STO), (c) andSTO Nb-STO, and Nb-STO, and (d )and amorphizable (d) amorphizable ceramics ceramics (YIG, LiNbO (YIG,3, LiNbO ZrSiO43,, andZrSiO GGG).4, and GGG). The likely process of nanostructure formation in fluorides (CaF2, SrF2, and BaF2) irradiated with 4.200 Conclusions MeV Au is presented in Figure8b. According to our previous study [ 36], most of the crystalline hillocks in these fluorides are nearly semispherical, although some of the hillocks have nearly spherical Amorphizable ceramics such as LiNbO3, ZrSiO4, and Gd3Ga5O12 were irradiated with 200 MeV shape. Since recrystallization plays an important role in both fluorides and CeO2, the formation process Au ions at an oblique incidence angle. Line-like homogeneous ion tracks and bell-shaped hillocks are in the fluorides should be similar to that of CeO2 (Figure8a). The di fference between the spherical observedshape of hillocksby TEM. and The non-spherical ion track and shape hillock can diameter be explaineds are similar by the for diff allerence the amorphizable in volume of protrusion. ceramics, although the hillock diameter is found to be slightly larger than the ion track diameter. The TEM images of ion tracks in STO and Nb-STO irradiated with 200 MeV Au show similar features to those observed in the non-amorphizable ceramics. For example, the ion track diameter is markedly smaller than the hillock diameter, and they exhibit inhomogeneity. However, the hillocks in these ceramics are found to be amorphous which is in contrast to the crystalline feature of hillocks observed in the non-amorphizable ceramics. Therefore, it can be concluded that STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics. No marked difference is observed between hillock formation in STO and that in Nb-STO. The material dependence of nanostructure formation can be ascribed to the intricate recrystallization process. The present results support that (1) simplicity of lattice structure and (2) the strength of ionic bonding can be the factors that determine the recrystallization effectiveness.

Funding: Part of the present work was financially supported by JSPS KAKENHI Grant Number 20K05389.

Acknowledgments: The authors are grateful to the technical staff of the tandem accelerator at JAEA-Tokai for supplying high-quality ion beams. One of the authors (N.I.) thanks A. Iwase and A. Kitamura for their constant support during the research.

Conflicts of Interest: The authors declare no conflict of interest.

References

Quantum Beam Sci. 2020, 4, 43 11 of 14

If the volume of protrusion is large, the hillock shape tentatively has an unstable shape because of its high aspect ratio of height to width. It is conceivable that the unstable shape can rapidly turn into a stable spherical shape. Similar spherical hillocks have been already reported in some ceramics (Gd2Zr2O7 [50] and YIG [51]) irradiated with high-energy fullerene ions with very high Se, supporting the proposition that larger volume of the protrusion is the key to the formation of spherical hillocks. The likely process of nanostructure formation in STO and Nb-STO is presented in Figure8c. Although the first and second steps of the process in the figure are the same as those in Figure8b, the third step of the process is different. Even though the molten region embedded in the matrix recrystallizes partially, the protruded part of the melt failed to recrystallize, resulting in amorphization. The failure of recrystallization only in the hillock region can be ascribed to oxygen deficiencies as discussed above. The likely process in amorphizable ceramics (YIG, LiNbO3, ZrSiO4, and GGG) is presented in Figure8d. The process consists of the following three steps. (1) A molten region is created along the ion path. A part of the molten region protrudes above the surface. (2) During cooling, the molten region embedded in the matrix starts to solidify, resulting in the amorphization of the molten region. Herein, the size of the molten region corresponds to that of the ion tracks. (3) The protruded part of the melt also starts to amorphize. This means that the hillock diameter is always similar to the ion track diameter. A subtle change of hillock shape leading to a slightly larger hillock diameter than the ion track diameter is not specified in the schematic, since this effect produces only a small difference between the hillock diameter and ion track diameter.

4. Conclusions

Amorphizable ceramics such as LiNbO3, ZrSiO4, and Gd3Ga5O12 were irradiated with 200 MeV Au ions at an oblique incidence angle. Line-like homogeneous ion tracks and bell-shaped hillocks are observed by TEM. The ion track and hillock diameters are similar for all the amorphizable ceramics, although the hillock diameter is found to be slightly larger than the ion track diameter. The TEM images of ion tracks in STO and Nb-STO irradiated with 200 MeV Au show similar features to those observed in the non-amorphizable ceramics. For example, the ion track diameter is markedly smaller than the hillock diameter, and they exhibit inhomogeneity. However, the hillocks in these ceramics are found to be amorphous which is in contrast to the crystalline feature of hillocks observed in the non-amorphizable ceramics. Therefore, it can be concluded that STO and Nb-STO are intermediate ceramics between amorphizable and non-amorphizable ceramics. No marked diﬀerence is observed between hillock formation in STO and that in Nb-STO. The material dependence of nanostructure formation can be ascribed to the intricate recrystallization process. The present results support that (1) simplicity of lattice structure and (2) the strength of ionic bonding can be the factors that determine the recrystallization eﬀectiveness.

Author Contributions: Conceptualization, N.I.; TEM observation, N.I. and T.T.; Sample preparation, N.I.; Ion irradiation, N.I. and H.O.; Manuscript writing, N.I. All authors have read and agreed to the published version of the manuscript. Funding: Part of the present work was financially supported by JSPS KAKENHI Grant Number 20K05389. Acknowledgments: The authors are grateful to the technical staff of the tandem accelerator at JAEA-Tokai for supplying high-quality ion beams. One of the authors (N.I.) thanks A. Iwase and A. Kitamura for their constant support during the research. Conflicts of Interest: The authors declare no conflict of interest.

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